refactor: use pid as sole background execution data field

Align with PR1's standardization on pid as the canonical field name.

packages/core/src/agents/remote-invocation.test.ts

@@ -657,7 +657,6 @@ describe('RemoteAgentInvocation', () => {
         'Remote agent moved to background',
       );
       expect(result.data).toMatchObject({
-        executionId,
         pid: executionId,
         initialOutput: 'Chunk 1',
       });

packages/core/src/agents/remote-invocation.ts

@@ -192,7 +192,6 @@ export class RemoteAgentInvocation extends BaseToolInvocation<
     const command = `${this.getDescription()}: ${this.params.query}`;
     const backgroundMessage = `Remote agent moved to background (Execution ID: ${executionId}). Output hidden. Press Ctrl+B to view.`;
     const data: BackgroundExecutionData = {
-      executionId,
       pid: executionId,
       command,
       initialOutput: output,